A. Definitions
As used in this specification and in the claims, the following terms shall have the following meanings:
(1) Telecentric
Telecentric lenses are lenses which have at least one pupil at infinity. In terms of principal rays, having a pupil at infinity means that the principal rays are parallel to the optical axis (a) in object space, if the entrance pupil is at infinity, or (b) in image space, if the exit pupil is at infinity.
In practical applications, a telecentric pupil need not actually be at infinity since a lens having an entrance or exit pupil at a sufficiently large distance from the lens' optical surfaces will in essence operate as a telecentric system. The principal rays for such a lens will be substantially parallel to the optical axis and thus the lens will in general be functionally equivalent to a lens for which the theoretical (Gaussian) location of the pupil is at infinity.
Accordingly, as used herein, the terms “telecentric” and “telecentric lens” are intended to include lenses which have a pupil at a long distance from the lens' elements, and the term “telecentric pupil” is used to describe such a pupil at a long distance from the lens' elements. For the projection lenses of the invention, the telecentric pupil distance will in general be at least about 20 times the lens' focal length.
(2) Effective Back Focal Length
The effective back focal length (BFL) of a projection lens/pixelized panel combination is the distance between the front surface of the pixelized panel and the vertex of the back surface of the rearward-most lens element of the projection lens which has optical power when (1) the image of the pixelized panel is located at infinity and (2) the projection lens is located in air, i.e., the space between the rearward-most lens element of the projection lens and the pixelized panel is filled with air as opposed to the glasses making up the prisms, beam splitters, etc. normally used between a projection lens and a pixelized panel.
(3) Q-Value
As described in J. Hoogland, “The Design of Apochromatic Lenses,” in Recent Development in Optical Design, R. A. Ruhloff editor, Perkin-Elmer Corporation, Norwalk, Conn., 1968, pages 6-1 to 6-7, the contents of which are incorporated herein by reference, Q-values can be calculated for optical materials and serve as a convenient measure of the partial dispersion properties of the material.
Hoogland's Q-values are based on a material's indices of refraction at the e-line (546 nanometers), the F′ line (480 nanometers), and the C′ line (643.8 nanometers). The wavelengths used herein, both in the specification and in the claims, are the d line (587.56 nanometers), the F line (486.13 nanometers), and the C line (656.27 nanometers).
More particularly, as described in Hoogland, the Q-value for a lens element is determined using the indices of refraction Nd, NF, and NC of the material making up the element at the d, F, and C lines, respectively, and the equation:Q=(y−yn)×106where y is given by:y=(NF−Nd)/(Nd−1)and yn is determined from an equation of the form:yn=ax+bevaluated at the x-value for the material making up the lens element, where x is given by:x=(NF−NC)/(Nd−1)and a and b are determined using x and y values for SK16 and SF2.
(4) V-Value
V-values (also known as Abbe constants) are for the d, F, and C lines and are given by:V=(Nd−1)/(NF−NC)
(5) Effective V-Value
The effective V-value (Ve) of one or more lens elements is given by:Ve=ΣVi·fiwhere the summation is over the one or more lens elements and Vi and fi are, respectively, the V-values and focal lengths of the individual lens elements.
(6) N-Value
Indices of refraction (N-values) are for the d-line (587.56 nanometers) in Table 9. All focal lengths and other calculated values which depend on a single value for the index of refraction for individual elements are for the e-line (546.1 nanometers).
(7) Vignetting
The vignetting of a projection lens in percent is defined as 100 minus 100 times the ratio, in the long conjugate focal plane, of the illuminance at the full field to the illuminance on-axis at the projection lens' working f-number. Since projection lenses normally do not include an adjustable iris and are used “wide open,” the working f-number will typically be the full aperture f-number.
B. Projection Systems
Image projection systems are used to form an image of an object, such as a display panel, on a viewing screen. Such systems can be of the front projection or rear projection type, depending on whether the viewer and the object are on the same side of the screen (front projection) or on opposite sides of the screen (rear projection).
FIG. 11 shows in simplified form the basic components of an image projection system 17 for use with a pixelized imaging device (also known in the art as a “digital light valve”). In this figure, 10 is an illumination system, which comprises a light source 11 and illumination optics 12 which transfer some of the light from the light source towards the screen, 13 is the imaging device, and 14 is a projection lens which forms an enlarged image of the imaging device on viewing screen 15. For front projection systems, the viewer will be on the left side of screen 15 in FIG. 11, while for rear projection systems, the viewer will be on the right side of the screen.
For ease of presentation, FIG. 11 shows the components of the system in a linear arrangement. For a reflective imaging device and, in particular, for a DMD imaging device of the type with which the present invention will typically be used, the illumination system is arranged so that light from that system reflects off of the imaging device, i.e., the light impinges on the front of the imaging device as opposed to the back of the device as shown in FIG. 11. Also, for such imaging devices, one or more prism assemblies (see “PR” in FIGS. 1–10) will be located in front of the imaging device and will receive illumination light from the illumination system and will provide imaging light to the projection lens. In addition, for rear projection systems which are to be housed in a single cabinet, one or more mirrors are often used between the projection lens and the screen to fold the optical path and thus reduce the system's overall size.
The linear arrangement shown in FIG. 11 can also be modified in the case of a transmissive imaging device. Specifically, in this case, the optical path between the imaging device and the screen can include two folds to reduce the overall size of the cabinet used to house the system, e.g., a first fold mirror can be placed between imaging device 13 and projection lens 14 and a second fold mirror can be placed between the projection lens and screen 15.
Image projection systems preferably employ a single projection lens which forms an image of: (1) a single imaging device which produces, either sequentially or simultaneously, the red, green, and blue components of the final image; or (2) three imaging devices, one for red light, a second for green light, and a third for blue light. Rather than using one or three imaging devices, some image projection systems have used two or up to six imagers. Also, for certain applications, e.g., large image rear projection systems, multiple projection lenses are used, with each lens and its associated imaging device(s) producing a portion of the overall image. Irrespective of the details of the application, the projection lens generally needs to have a relatively long effective back focal length to accommodate the prisms, beam splitters, and other components normally used with pixelized panels. In the preferred embodiments of the present invention, a single projection lens is used to form an image of a single imaging device, e.g., a DMD panel. For this application, the projection lens needs to have a relatively long effective back focal length to accommodate the one or more prism assemblies used with such a panel.
A particularly important application of projection systems employing pixelized panels is in the area of rear projection systems which can be used as large screen projection televisions (PTVs) and/or computer monitors. To compete effectively with the established cathode ray tube (CRT) technology, projection systems based on pixelized panels need to be smaller in size and lower in weight than CRT systems having the same screen size.